1. Field of the Invention
This invention relates to the fields of image compression and transmission.
2. Description of the Related Art
The flow of information over communication channels such as the Internet and cellular services is increasing rapidly. Unfortunately, channels such as these tend to be noisy. When using wireless services, for example, data bits conveying information can often become flipped or corrupted. With the Internet, a bitstream conveying information is divided into packets, some of which may be delayed or lost en route to their destination. Complicating the situation is that dataxe2x80x94particularly graphical dataxe2x80x94conveyed over such bandwidth-limited channels is invariably compressed, which increases the adverse affect of lost bits or packets.
A number of standards are in use or are under development that dictate how various pieces of information, such as still images, video, or audio, are to be compressed and transmitted. The proposed MPEG-4 standard, for example, defines a number of xe2x80x9cmedia objectsxe2x80x9d xe2x80x94including still images, video objects, and audio objectsxe2x80x94and prescribes how each object type is to be encoded and decoded. In some instances, the way in which an object is required to be handled increases the importance of a reliable communication channel. The MPEG-4 standard with respect to still images, for example, requires the use of an arithmetic coder (AC) when encoding an image for transmission. Use of an AC provides state-of-the-art compression; however, this coding scheme is very sensitive to channel error propagation.
Various techniques are available to reduce sensitivity to channel error propagation, but each has corresponding disadvantages. For example, an AC could be replaced with a fixed length entropy coder (FLC), but this results in a degradation of coding efficiency that may be unacceptable. Another technique uses an xe2x80x9cerror resilient entropy codexe2x80x9d (EREC), as described in D. W. Redmill and N. G. Kingsbury, xe2x80x9cEREC: an Error-Resilient Technique for Coding Variable-Length Blocks of Dataxe2x80x9d, IEEE Transactions on Image Processing, Vol. 5, No. 4, April 1996, pp. 565-574, for example, requires major changes in the coding algorithm; scalability is also adversely affected.
A packetization method and packet structure are presented which provide a way of transmitting compressed still images over noisy communication channels with improved error resilience. The method does not affect spatial or quality scalability, and a relatively small impact on coding efficiency.
The invention is useful when it is desired to improve the robustness of a bitstream generated when a still image is decomposed with a wavelet transform, regardless of the scanning method used to quantize the wavelet coefficients that result from the decomposition. After the wavelet decomposition, the wavelet coefficients of one xe2x80x9ctexture unitxe2x80x9d are scanned and coded in accordance with a chosen scanning method to produce a bitstream. The bitstreams for an integral number of texture units are assembled into a packet, each of which includes a packet header. Each packet header includes a resynchronization marker, the use of which enables a decoder to resynchronize with the bitstream if synchronization is lost, and an index number, which identifies one of the texture units in the packet and thereby enables a decoder to associate following packets with their correct position in the image.
The invention described herein enables a channel error to be localized to a particular packet, which allows the corrupted data to be discarded and prevents the effects of the error from propagating beyond packet boundaries. It also enables the decoder to resynchronize with the encoder and to associate the un-corrupted data that follows the error-containing packet to its correct position in the image. This is accomplished without degrading the spatial and quality scalability inherent in the encoding scheme, and while some overhead is inevitably required to practice the method, the impact on coding efficiency is relatively small.
The invention is applicable to the MPEG-4 and JPEG-2000 image compression standards currently under development, each of which uses an arithmetic coder (AC). When an AC is used in the encoding process, the inventive method requires that the statistical models used by the AC are reset at the start of each packet.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.